1. Field of the Invention
The present invention relates to apparatus used in substrate processing, and more specifically, to a substrate holder used to support a substrate in a vertical position during the performance of a process on the surface(s) of the substrate.
2. Description of the Prior Art
The surfaces of substrates are routinely subjected to various processes in the production of LSIs (large scale integrated circuits), LCDs (liquid crystal displays), and data storage disks. In the production of data storage disks of the type used in "hard disk" drives, a substrate holder may be used to vertically support a substrate having a hole formed in its middle. This permits a processing step, e.g., thin film deposition, to be efficiently performed on both sides of the substrate.
FIG. 6 is a front view showing a prior art substrate holder 50. FIG. 7 is an isometric view which illustrates the operation of substrate holder 50 shown in FIG. 6. Substrate holder 50 shown in FIGS. 6 and 7 includes a vertical base plate 11, fixed supporting claws 12, and a movable claw 13, with claws 12 and 13 connected to base plate 11.
As shown in FIGS. 6 and 7, base plate 11 is shaped in the form of a rectangular plate and is provided with a "J"-shaped cut-out. The curved portion of this "J"-shaped cut-out (referred to as the "curved part") is typically provided with four fixed support claws 12. As FIGS. 6 and 7 show, fixed support claws 12 are disposed at positions arranged symmetrically about a vertical line 10 passing through the lowest point of the "J" (referred to as "central axis 10").
Each of fixed support claws 12 is an "L" shaped member formed by bending a thin ribbon-shaped piece of metal or similar material. Notches are provided in the cut-out portion of base plate 11, and fixed support claws 12 are each disposed within the notches. The support claws 12 are fixed to base plate 11 with screws. The end of each fixed supporting claw 12 points toward the center of curvature of the curved part, and "V"-shaped indentations are formed at their distal ends as shown in FIG. 7.
Movable support claw 13 is fixed to the upper surface of base plate 11. Movable support claw 13 is also a ribbon-shaped member bent into an "L" shape. One end of movable claw 13 is fixed to the upper end surface of base plate 11 with screws, with the distal tip portion positioned directly on central axis 10 and pointing downward. A "V"-shaped indentation is formed in the downward distal end of moveable claw 13 in the same manner as for fixed support claws 12.
The operation of the above mentioned conventional substrate holder 50 will now be described. Substrate holder 50 supports a substrate 2 which is a flat disk with a hole 20 at its center, e.g., a hard disk substrate. Substrate 2 is placed in and removed from substrate holder 50 by a transfer mechanism 4 equipped with a substrate pick-up 3 that holds substrate 2 (see FIG. 7).
As FIG. 7 shows, an opening/closing device 5 which actuates movable support claw 13 is positioned near substrate holder 50. Movable support claw 13 acts as a leaf spring, and opening/closing device 5 includes an opening/closing pin 51 which causes movable support claw 13 to bend by displacing the end of movable support claw 13 upwards. Opening/closing device 5 also includes a moving mechanism 52 which causes pin 51 to displace the end of claw 13 laterally toward or away from substrate holder 50 to engage or disengage pin 51 from claw 13.
The way in which a substrate 2 is loaded on substrate holder 50 will now be described. After substrate pick-up 3 engages substrate 2 by inserting its tip into hole 20 in the center of substrate 2, the substrate is transferred horizontally to the vicinity of substrate holder 50 by transfer mechanism 4. Pin 51 of opening/closing device 5 is moved horizontally (i.e., laterally) by moving mechanism 52 to a position below movable support claw 13 of substrate holder 50. Pin 51 moves up, causing movable support claw 13 to bend and be displaced upward, as shown by the dotted line in FIG. 6. This opens movable support claw 13.
Transfer mechanism 4 then moves substrate 2 into alignment with base plate 11 such that substrate 2 is positioned between movable support claw 13 and fixed support claws 12. Transfer mechanism 4 then causes substrate pick-up 3 to descend slightly, such that substrate 2 rests on and is supported by fixed support claws 12.
Moving mechanism 52 of opening/closing device 5 then causes pin 51 to descend, such that movable support claw 13 is returned to an approximately horizontal position (the solid line in FIG. 6). This causes the end of movable support claw 13 to contact the upper edge of substrate 2, pressing down on substrate 2 from above. Substrate pick-up 3 then withdraws from substrate holder 50 and is returned to a standby position by transfer mechanism 4, ready to pick up another substrate. Pin 51 is then disengaged from moveable claw 13 and is also moved to a prescribed "standby" position.
When extracting substrate 2 from substrate holder 50 the opposite procedure is performed. Moving mechanism 52 of opening/closing device 5 causes pin 51 to move to a position below movable support claw 13, and then lifts pin 51, displacing the end of movable support claw 13 and disengaging it from the substrate. Transfer mechanism 4 inserts the end of substrate pick-up 3 into hole 20 in the center of substrate 2. Substrate pick-up 3 rises up slightly to hold and disengage substrate 2 from support claws 12. Transfer mechanism 4 then withdraws substrate 2 away from substrate holder 50.
FIG. 8 is a plan view of an exemplary substrate processing device which uses substrate holder 50 of FIGS. 6 and 7. The substrate processing system shown in FIG. 8 includes a substrate loading/unloading chamber 61 and an adjacent processing chamber 62 separated by gate valve 66. Chamber 61 may be a load lock chamber. A substrate 2 to be processed is loaded on substrate holder 50 inside chamber 61. Each substrate 2 is supplied to process chamber 62 on a separate substrate holder 50. After substrate 2 has been processed in processing chamber 62, substrate holder 50 is returned to chamber 61, where it is removed from substrate holder 50.
As an example of a substrate processing device, FIG. 8 illustrates a sputtering device which produces magnetic thin films. Two sputtering sources 63 are provided inside of processing chamber 62. Sputtering sources 63 each consist of a sputter target 631 and a magnet assembly 633 provided at the rear of target 631. A sputtering power source 632 which applies a prescribed voltage to target 631 is also provided.
Magnet assembly 633 comprises a central magnet 634 and a ring-shaped peripheral magnet 635 which surrounds central magnet 634. Magnet assembly 633 establishes arch-shaped magnetic flux lines 636 passing through target 631. When a voltage is applied to the target at low pressure, magnetic flux lines 636 give rise to magnetron sputtering.
Process chamber 62 is equipped with a vacuum pumping system 64 and process gas supply system 65 for the introduction of a process gas into its interior. When sputtering power source 632 is operated while introducing the process gas into chamber 62, a magnetron sputtering discharge occurs in the space adjacent to the surface of target 633, and a magnetron plasma, P, is formed in the regions between the sources 63 and the substrate.
As FIG. 8 shows, the pair of sputtering sources 63 are provided on both sides of substrate 2 which is supported by substrate holder 50. Accordingly, magnetron plasma P is formed on both sides of substrate 2, and film deposition is performed simultaneously on the surfaces of both sides of substrate 2.
It is desired that a substrate processing system, such as the one described, be capable of producing highly uniform films on the entire surface of the substrate. For example, in a process to make magnetic thin films on substrates for magnetic recording media, the processing device should produce a high-quality magnetic thin film on both sides of the substrate that extends as close as possible to the outer edge of the substrate in order to increase the recording capacity of the disk.
However, in research conducted by the present inventors, it was determined that when a substrate holder of the type described with reference to FIGS. 6-7 is used, although a high-quality magnetic thin film can be made in the central portion of the substrate, the film is not uniform across the entire surface of the substrate. In particular, it was determined by the inventors that films produced using substrate holder 50 have localized defects (i.e., non-uniform film quality) in regions near the edge of the substrate.
FIGS. 9A and 9B are graphs which illustrate the problem caused by using a conventional substrate holder which is remedied by the present invention. FIGS. 9A and 9B show the modulation characteristics of a magnetic thin film produced using substrate holder 50 shown in FIGS. 6 and 7. The modulation characteristics indicate whether or not there are abnormalities in the modulation (i.e. output signal) of a signal read back from a substrate. In the present case, they show whether or not the magnetic characteristics of the magnetic thin film produced on the substrate are uniform in the area at the periphery of the substrate. Note that FIGS. 9A and 9B represent the magnitudes of the coercive force and remnant magnetic flux in the circumferential direction, with those characteristics being represented by an electrical signal.
FIG. 9A is a graph showing the characteristics of a magnetic thin film deposited on a substrate 95 mm in diameter using the conventional substrate holder of FIGS. 6 and 7 at a radial distance of 40 mm from the substrate center. FIG. 9B is a graph showing the magnetic characteristics of a magnetic thin film deposited on a substrate 95 mm in diameter using the conventional substrate holder of FIGS. 6 and 7 at a radial distance of 46.5 mm from the substrate center. As the figures show, the magnetic characteristics are uniform at 40 mm, because the modulation curve is essentially constant. However, at 46.5 mm from the center, unwanted modulation occurs as indicated by peaks or localized jumps in the magnetic characteristics (which are observed at five places in the graph) showing that the film is not uniform. Occurrence of this sort of modulation is likely to result in errors during magnetic recording, and is a source of bad sectors on the storage medium.
After making a detailed study of the factors which might be responsible for causing these modulation peaks, the present inventors concluded that the positions of the peaks correspond to the positions of the support claws of the substrate holder shown in FIG. 6. That is, the positions of the peaks of FIG. 9B coincides with the positions of the support claws. The inventors have concluded that the support claws are responsible for the observed non-uniformity in the magnetic characteristics of the film.
The inventors have determined that the nonuniformity caused by the support claws is due to the influence of the support claws on the plasma formed during film deposition. In a conventional substrate holder 50 shown in FIG. 6, the plasma enters the space or gap between the edge of the cut-out in the base plate of the substrate holder and the outer edge of the substrate. The entry of plasma becomes a problem when a bias voltage more positive than the sputter cathode voltage is applied to the substrate.
Since the support claws are normally formed of a stainless steel or other metal, electrons and negative ions from the plasma flow freely to the grounded parts of the substrate holder through the support claws (e.g., when the support claws are held at ground potential). The flow of charged particles out of the plasma in the vicinity of the support claws causes local plasma changes in the characteristics of the plasma, with the plasma variation occurring where the support claws are present. This local change in the plasma characteristics produces the observed localized variations in the magnetic characteristics of the film deposited on the substrate.
A study was performed by the inventors involving the application of a negative bias voltage to the substrate to draw positive ions from the plasma for use in processing of the substrate. In the study, the supporting claws acted as electrodes for applying the negative bias voltage to the substrate. This study confirmed that when plasma enters into the gap between the substrate edge and the base plate, the characteristics of the plasma are significantly changed by the presence of the support claws, and the unwanted localized variation of magnetic characteristics of the type shown in FIG. 9B will occur. This has a detrimental effect on the data storage ability of the affected region of the disk, and acts to reduce the overall storage capacity of the disk by reducing the usable area of the disk.
What is desired is a substrate holder that allows disposition of films with a uniform quality over the entire surface of a substrate, i.e., without the localized variation of characteristics which arise from the presence of the support claws which are found in the prior art.